This invention relates to ring laser angular rate sensors. In particular, it relates to the avoidance of lock-in in such sensors by moving at least one mirror which defines the ring laser optical closed-loop path in such a manner so as to cause the average error produced by wave scattering effects to be reduced to zero.
Reference is hereby made to U.S. Pat. No. 3,373,650 entitled, "Laser Angular Rate Sensor" and U.S. Pat. No. 3,467,472 entitled, "Random Bias For A Ring Laser Angular Rate Sensor" both by J. E. Killpatrick, and U.S. Pat. No. 3,390,606 entitled, "Control Apparatus" and U.S. Pat. No. 4,152,071 entitled, "Control Apparatus" both by T. J. Podgorski, all of which are assigned to the same assignee as the present application. Reference is also hereby made to a publication entitled, "The Laser Gyro" by Frederick Aronowitz, Laser Applications Vol. 1, 1971.
The references cited above illustrate the principles and problems of ring laser angular rate sensors well known in the art. A ring laser angular rate sensor usually comprises two countertraveling waves or beams in the form of laser beams which propagate about an optical closed-loop path defined by a plurality of wave reflecting means or mirrors. In such sensors there exists a phenomenon known as lock-in which is primarily attributed to wave or light scattering, at the mirrors, of the impinging wave. The light scattering causes the phenomenon known as lock-in which is a major source of angular rate information error produced in such sensors.
In the aforementioned patents, a method known as dithering is utilized to reduce the effects of lock-in. These techniques are well known and have their advantages and disadvantages well known in the art. Such dithering techniques utilize the concept of providing an alternating bias to the frequency difference between the counterpropagating waves. Doing so allows a marked improvement in ring laser angular rate sensors which enhances their capabilities for obtaining low error angular rate information.